<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #box1{width: 300px;height: 300px; border: black 1px solid; position: relative;}
        #box2{width: 100px;height: 100px; background:burlywood;
        position: absolute; left :50%; top:50% ; margin: -50px 0 0 -50px;}
        /* 在父容器中定位居中 */
    </style>
</head>
<body>
    <div id="box1">
        <div id="box2"></div>
    </div>
</body>
</html>